Cat not eating, drinking, or playing

Hi, I'm new here. I came because of a problem with my 14 year old tuxedo cat, Lisa, who loves to eat. But yesterday I noticed she was not eating her food or drinking at all. We fed her some turkey and a few treats, which she ate. That was all she ate yesterday. She had slept almost all day and didn't play with string. Today, we are taking her to the vet. I wanted to take her in the morning but we may have to wait until 6:00 PM. (it is 10:00 AM) She had diarrhea (soft and yellow;no blood) and didn't eat or drink. We fed her a few bites of some wet food which she slowly ate. Afterwards she threw up twice, which was basically brown water. She has also been lethargic. I am very worried because she never turns down food, and is not picky at all. There are countless things I read about that share her symptoms.  Here are her symptoms:

Diarrhea
Loss of appetite
Dehydration
Lethargy
Vomiting

She also had runny eyes and nose yesterday, but not today.

I'm not 100% sure she is dehydrated. I pulled her scruff and it went down relatively slowly, so I think she is pretty dehydrated. Her breathing sounds normal. I'm not sure if its worth noting, but shes pretty overweight. (since, like I said earlier, she love love loves to eat)

I'm wondering if diarrhea is the base of the problem (which I am hoping) and the others are symptoms, or if diarrhea is a symptom and the base of the problem is something else. I really appreciate any help!

You are right it could be any number of things so it's good that she is going to the vet.  One thing you need to be aware of is fatty liver which cats can get when they don't eat.  Overweight cats are particularly vulnerable to it.  It happens because the cats liver is not very efficient with metabolizing fat stores.  The body tries to use them, the liver can't keep up and fat deposits on the liver.  You may need to syringe feed her.  There are several videos on youtube that shows how to do it.  Basically you use the side of her mouth, go very slow and use a smooth pate type food.l

She needs to get to the vet ASAP.

Those are worrying symptoms, and I hope the vet can give you some answers. Please let us know what happens!

She tried to drink a second ago but looked very uncomfortable. She was huffing and sneezing when she drank and she just dipped her mouth in and gulped instead of lapping it up.

Also, sorry about the dumb question but, we don't have a syringe and I really want her drinking. We can't use a straw to make her drink, can we?

Something is definitely going on with your kitty, please get her to the vet as soon as possible.  I do not know how you could give her water through a straw.